Setting up your own certificate authority on IIS7 using OpenSSL and securing your web api with client certificates

Creating self signed certificates isn’t really all that complicated, but it can be a little intimidating the first time you do it.

What are we trying to achieve?

1) A web api that is protected by client certificates hosted on IIS7.
2) A way to test it out from our browser.

Why?

Well, the use case is a web api that is not open to the public. We want to secure it such that only clients with the relevant certificates can access the api.

Pre-requisites

A web site that is protected by a valid SSL certificate
OpenSSL – I installed this on a 64bit version of Win7. Be warned, you need Visual C++ 2008 redistributable installed first. Be warned, yet again, that even though open ssl should install in 64bit mode, I couldn’t get it working so I just took the 32 bit. Hasn’t done me any harm….. 🙂

Gotchas

Testing your api from a browser like IE requires you to have a p12 client certificate to import into your personal certificate store. This one caught me out for a while.

Setting up your root certificate authority

First create a key pair that you will use to sign your certificate:

openssl genrsa -des3 -out root-ca.key 1024

Enter a strong pass phrase. This is the most vital pass phrase you will ever come up with – your root certificate is what you will use to sign client certificates and it is what will be installed on your IIS7. Basically, if someone gets your root cert and your passphrase for it, they can create their own client certificates and your web api will trust them. If you are super paranoid, disconnect the server that you are using to create this certificate from the network forever.

Now use your key pair to create and sign a root certificate:

openssl req -new -x509 -days 3650 -key root-ca.key -out root-ca.crt

We are generating a certificate that will be valid for 10 years. Make it shorter if you prefer. You’ll be prompted for your root key pair pass phrase and a bunch of info – fill it in, forget about the email address.

You now have a root certificate – this is what you will install on your web server. You will also use the root certificate to sign client certificate requests. Once the client certificate request has been signed by with the root certificate, any requests to your secured api with these client certificates will be implicitly trusted by your web server.

Make your webserver recognise your new certificate as a trusted Certificate Authority

Create a website and require certificates

Create a new site or application in IIS, and then using the IIS manager, select the SSL Settings.

Make sure Require SSL is checked and that the Client Certificates option is set to Require.

If you try to browse your website now, you should get an access is denied message.

Create a client certificate request

You’ve got your root cert. You’ve installed it on your webserver. You’ve locked down your website. All that’s left to do is create the client certificate, install it in your certificate store on the client machine and away you go.

Open up the command prompt on your client machine which has openssl installed on it and:

openssl genrsa -out client-cert.key 1024

As above, we generate a keypair, and then create the certificate request:

openssl req -new -key client-cert.key -out client-cert.csr

Again, you’ll be prompted for all sorts of information – fill it in. When you put the organisation name and common name, use something different from your root certificate above so you can keep tabs on things in your personal certificate store.

Now, we use the client certificate request and create a client certificate, signing the certificate with our root certificate:

You’ll be asked for your root certificate pass phrase – you remember, the one I told you was super-important above.

Very cool, we now have a client certificate that IIS will trust because it has been signed by a root certificate that IIS trusts. Woohooo.

And here is the gotcha – If you simply install this .crt in your certificate store and try to browse your locked down website, it just will not work!

And here is the fix – see Internet Explorer needs the certificate to be in a specific format (pkcs12) for it to actually present the certificate to the webserver when you go a-browsing. Luckily, openssl allows us to fix this issue:

Again, double click the .p12 file, or go through mmc to import the p12 certificate and you should be away. Close IE, re-open and browse to your locked down website. You should be greeted with your website.